1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for generating, acquiring and processing land seismic data and, more particularly, to mechanisms and techniques for separating seismic signals generated by plural land seismic sources.
2. Discussion of the Background
Land seismic data acquisition and processing may be used to generate a profile (image) of the geophysical structure under the ground (subsurface). While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs. Thus, providing a high-resolution image of the subsurface is important, for example, to those who need to determine where oil and gas reservoirs are located.
Geophysical prospectors have found seismic vibrators to be useful signal sources for imaging the earth. Conventional seismic acquisition in the past generally employed multiple vibrators acting together and initiated simultaneously to form a source array. In land-based operations, the vibrators are positioned at a source location and synchronized to the same pilot sweep signal. Once activated, the vibrators generate a sweep that typically lasts between five and twenty seconds and typically spans a predetermined range of frequencies. A recording system that is connected to a plurality of receivers, typically geophones for land-based seismic exploration, is employed to receive and record the response data. For reflection seismology, the record length is typically set to equal the sweep length plus a listen time equal to the two-way travel time, which is the time required for the seismic energy to propagate from the source through the earth to the deepest reflector of interest and back to the receiver. The vibrators are then moved to a new source location and the process is repeated.
The conventional methods have a number of shortcomings some of which include: 1) intra-array statics because the vibrators are at different elevations or variations in the near surface that can affect source coupling to the earth; 2) spatial resolution issues due to array effects and limitation in source effort because of economic constraints; 3) control and synchronization problems associated with the use of multiple sources; and 4) mixed-phase data produced by the correlation process. Improvements in technology and reductions in the per channel cost of recording have resulted in an industry push toward using point source-point receiver methods to overcome some of the problems associated with source arrays and large receiver arrays. In land surveys today, the use of point receivers has rapidly increased productivity in deploying the receiver spread. As a result, the vibrators have become the weak link in achieving efficient field operations.
Over the years a number of methods have been introduced to address shortcomings with conventional seismic survey methods. One method, titled “Method for Continuous Sweeping and Separation of Multiple Seismic Vibrators,” by Krohn and Johnson (WO/2005/019865), the entire disclosure of which is incorporated herein by reference, attempts to address the data quality and data acquisition issues. This method is an extension of the High Fidelity Vibratory System (“HFVS”) originally developed by MOBIL and ARCO (see U.S. Pat. Nos. 5,719,821 and 5,721,710). The MOBIL-ARCO alliance developed a data acquisition and data processing technique that eliminates vibrator intra-array statics problems, mitigates vibrator control errors, provides minimum phase data, and provides high spatial resolution. However, in order to provide a cost effective method for effectively collecting point source data, a means to separate vibrators sweeping simultaneously was necessary.
Continuous HFVS is a new EXXON-MOBIL technique that combines a variation of the EXXON-MOBIL Cascaded Sweep technique and HFVS. By linking sweeps together with no listen time between segments, recording acquisition time is reduced. For example, a four vibrator implementation with four 12 second sweep segments and a five-second listen time and collected in one record has a total duration of 53 seconds. Compared to the 68-second duration in HFVS, this approach can result in a 22 percent timesaving. In practice, the record is processed by dividing the longer record into shorter records, and then conventional HFVS processing is performed.
However, the HFVS-methods have some technical drawbacks including: 1) low frequency noise in the inverted records because of an absence of low frequencies in the source signal; 2) not capturing all harmonic energy produced in the measured source signal; 3) a large data volume; 4) poor quality control because uncorrelated data is used (“shooting blind”); and 5) source outputs using phase offset encoding, resulting in highly correlated source signals that require reliable phase encoding for good separation.
The cost of seismic surveys depends heavily on the time required to collect the data. To reduce the acquisition time a number of methods have been devised over the years. Methods for source separation disclosed vibrator sources that are operated concurrently to reduce the time required for acquiring seismic survey data. For example, two groups of vibrators shooting into the same receiver spread at different offsets can be used to form a composite record. Most of those methods involve some form of swept sine wave source signal and rely on properties of the sweeps to be separated by correlation. Some methods rearrange portions of a conventional swept sine wave to mitigate crosstalk between surveys due to cross-correlation between the sweeps employed (see U.S. Pat. Nos. 4,168,485 and 4,982,374). Others achieve separation by using phase encoding schemes sometimes combined with up-sweeps and down-sweeps (see U.S. Pat. No. 4,823,326), others use time delays (see U.S. Pat. No. 4,953,657) and still others employ different sweep rates (see WO 2008/025986). Still others use techniques such as slip-sweep (see U.S. Pat. No. 6,603,707) that combine conventional swept sine waves, time delayed starts, and processing methods of F-T filtering, deconvolution, and migration to achieve separation (also see WO 2006/018728).
One such method, disclosed in U.S. Pat. No. 7,859,945 (herein '945), the entire content of which is incorporated herein by reference, implements a seismic acquisition using vibrators operating simultaneously. In this way, the time spent for the seismic survey is further reduced. The sweeps employed by the vibrators are based upon modified pseudorandom digital sequences. These sweep modifications include: 1) spectral reshaping, 2) cross-correlation suppression over a time window of interest, and 3) level compression to restore a favorable root mean square (“RMS”) to peak amplitude level. The composite received signal reflected from the subsurface formations is correlated with the pilot sweep signals for preliminary separation. The individual responses of subsurface formations in the transmission path between each individual source and each seismic detector may be recovered, with the source signature removed.
However, there is a need to develop a method that is capable of simultaneously using multiple seismic vibrators while also reducing the constraints on the system configuration, e.g., relaxing starting times of the various vibratory sources.